This invention relates to high resolution sensing of the position of a linearly movable element and, more particularly, to apparatus for high resolution sensing of the position of a rapidly moving element such as a tuning shaft of a frequency agile magnetron.
It is frequently necessary to sense the position of a rapidly moving element with a high degree of resolution. One example of such a requirement occurs in the tuning of a frequency agile magnetron. The resonant frequency of the magnetron is rapidly varied by driving a tuning plunger at a high rate of speed. As the tuning plunger is driven, it is necessary to know the resonant frequency of the magnetron with high resolution. In general, the tuning plunger has a small total travel, typically less than one inch, but moves very rapidly, typically 0.25 inch/millisecond. To sense the position of the tuning plunger with high resolution requires a fast-acting sensing system. For example, when the speed is 0.25 inch/millisecond and a resolution of 0.005 inch is required, a minimum of 50,000 position samples per second must be taken.
Optical encoders in which an optical system counts lines on a moving piece of glass can achieve the required resolution. However, such optical encoders are too expensive in many applications, are subject to errors and create packaging problems, particularly where the equipment is required to operate in adverse environmental conditions.
Linear variable differential transformers (LVDT's) have also been used to accurately sense the position of a linearly movable element. The LVDT is an electromechanical device that produces an electrical output proportional to the displacement of a separate movable core. An ac voltage is applied to a primary coil. The voltage induced in two secondary coils depends on the position of the movable core, which is attached to the movable element of interest. The voltage induced in the secondary coils is sampled every cycle or every half cycle to provide sensing of the core position. LVDT's provide good linearity and high resolution when the core speed is relatively low. However, as the rate of travel of the core increases, the position resolution decreases for a given excitation frequency. The excitation frequency can be increased but, at higher excitation frequencies, the performance of the LVDT is degraded. Typically, the excitation frequency is limited to about 20 kilohertz, corresponding to a full-wave sampling rate of 40,000 samples per second. Therefore, in the past, LVDT's have been limited in their application to relatively low rates of core travel.
It is, therefore, an object of the present invention to provide novel high resolution position sensing apparatus.
It is another object of the present invention to provide apparatus for high resolution sensing of the position of a movable member during high speed linear translation thereof.
It is yet another object of the present invention to improve the resolution of position sensing apparatus incorporating linear variable differential transformers.